1. Field of the Invention
The present invention relates to light sources for illumination, and more particularly to light-emitting devices and luminaires having refracting and reflecting surfaces to alter the distribution of light from a luminous element.
2. Description of the Related Art
Light-emitting elements (LEEs) are ubiquitous in the modern world, being used in applications ranging from general illumination (e.g., light bulbs) to lighting electronic information displays (e.g., backlights and front-lights for LCDs) to medical devices and therapeutics. Solid state lighting (SSL) devices, which include light-emitting diodes (LEDs), are increasingly being adopted in a variety of fields, promising low power consumption, high luminous efficacy and longevity, particularly in comparison to incandescent and other conventional light sources. A luminaire is a lighting unit that provides means to hold, position, protect, and/or connect one or more light-emitting elements to an electrical power source, and in some cases to distribute the light emitted by the LEEs.
One example of a LEE increasingly being used for in luminaires is a so-called “white LED.” Conventional white LEDs typically include a “pump” LED that emits blue or ultraviolet light, and a phosphor or other luminescent material. The device generates white light via down-conversion by the phosphor of a fraction of the blue or UV light from the LED (referred to as “pump light”) to light of a red, yellow, or green wavelength, or a combination of wavelengths longer than that of the pump light, and the mixing of light having these various wavelengths from the pump and phosphor. Such white LED devices are also referred to as phosphor-converted LEDs (pcLEDs). Although subject to some losses due to the light conversion, various aspects of pcLEDs promise reduced complexity, better cost efficiency and durability of pcLED-based luminaires in comparison to other types of luminaires. In conventional pcLEDs, a phosphor is often coated directly onto the semiconductor die of the pump LED or suspended within an encapsulant very close to the semiconductor die within the LED die package. In this position, the phosphor is subjected to the high operating temperature of the semiconductor die, and it is difficult to design and fabricate a die package with excellent angular color uniformity.
Recently, light sources have been introduced in which a phosphor is coated onto, or suspended within, a larger substrate that is separated spatially somewhat from the LED die package instead of incorporating it close to the die. Such a configuration may be referred to as a “remote phosphor.” Increasing the separation of the phosphor from the pump LED provides many potential benefits including better color control, higher system efficiency and luminous efficacy, lower operating temperature—leading to higher reliability (lumen maintenance) and color stability, design freedom to shape the emitting surface, and lower glare from a larger emitting surface. The remote phosphor approach is driving rapid advancement in high-power and high-efficiency solid-state light sources for general lighting. Together with the improvements in luminous efficacy that are associated with the LED light sources themselves, there is also a push for improved optics to direct the light where it is needed with high efficiency, which is another component of energy efficiency.
Large-area remote phosphor elements, often packaged together. e.g. with arrays of pump LEDs placed in so-called “mixing chambers,” behave as Lambertian emitting surfaces that are good for wide-angle ambient lighting when placed in simple luminaires having little or no secondary optics (i.e., optics outside of the LED package) for shaping the light distribution. But compared to the more point-like earlier-generation white LEDs, it is more difficult to design and manufacture efficient optics to tailor the light distribution from these extended-source luminous elements, especially for applications needing narrower beam patterns or steep cutoff at the beam edges. Certain conventional approaches to making narrower or steeper patterns result in unwanted losses in efficiency, as some light from the luminous elements is blocked rather than redirected into the desired beam pattern. Similarly, using conventional imaging optics to generate image-forming designs, even by experienced practitioners, generally results in systems of large size and having significantly less than the maximum possible optical efficiency. Currently, a relatively small proportion of designers are well versed in the young specialty of nonimaging optics, which, as the relevant discipline for light-emitting devices and luminaires for illumination, rigorously analyzes and enables designs achieving the maximum thermodynamic limits for efficiency. In addition, much of the nonimaging optics theoretical literature is related to light-collecting concentrators rather than to light-emitting devices. Design of optics for LEDs and luminaires thus lags somewhat behind the rapid advancements in high-power LED light sources themselves.
There is accordingly an ongoing need for novel light-emitting devices and luminaires designed to operate efficiently in conjunction with Lambertian extended sources such as those using remote phosphor luminous elements, using simple to fabricate refractive and/or reflective surfaces, and with the ability to tailor the far-field patterns to have useful profiles for a variety of general lighting applications.